Integrated circuit design generally involves producing large numbers of integrated circuits on a single silicon wafer through processes such as photolithography. The wafers are sawn into individual chips (i.e., die) that each contain a copy of the integrated circuit. Each die, or chip, is mounted on a substrate such as a ball grid array, or “BGA”. A BGA typically serves as the substrate or packaging for the die integrated circuit. A die is connected to the BGA either by wire bonding or through solder bumps on its bond pads when the die is “flip-chip” mounted. A “flip-chip” is a die that can be attached or bonded directly to a BGA in a “face-down” manner without any wire bonding. The flip-chip has pre-processed bond pads on which solder bumps are typically formed, enabling the face-down attachment of the flip-chip to contacts on the BGA through, for example, ultrasonic or reflow solder processes.
Solder joints in both flip-chip and wire-bonded die can crack due to various stresses such as cyclical mechanical and thermal loading on the BGA, the printed circuit board (PCB), and/or the die itself. Mechanical loading can be caused by vibration during transportation, for example, and thermal stresses occur during normal operation. Cracked solder joints can cause bond pads on the die to become detached or otherwise loosened from the BGA, resulting in open or intermittent electrical connections between the die integrated circuit and the BGA and/or a printed circuit board. In order to reduce the failures associated with cracked solder joints, functional bond pads are often replaced with dummy bond pads that are not used by the die integrated circuit. The dummy bond pads are typically located on the die in areas of higher stress such as corners, and/or in areas of greater temperature fluctuations. Therefore, if solder joints (either flip-chip or wire bonded) between dummy bond pads and BGA contacts become cracked, loosened, or otherwise damaged, performance of the integrated circuit on the die is not compromised.
However, damage to solder joints is not necessarily limited to the dummy solder joints in high stress areas of the die. Solder joints associated with functional bond pads can also become cracked, loosened, or otherwise damaged. For example, although a cracked solder joint may initially occur at the corner of the die, the cracking condition tends to propagate inward toward the center of the die, thereby compromising functional or non-dummy bond pads and resulting in open or intermittent electrical connections to the die integrated circuit.
Generally, these same types of stresses and damage to die solder joints also apply to solder joints between the BGA and printed circuit board. Thus, cracked and damaged solder ball joints between BGA and PCB contacts can likewise result in open or intermittent electrical connections to a die integrated circuit.